Hepatobiliary disease may complicate cystic fibrosis and lead to end-stage liver disease. The goal of this project is to prospectively follow patients with cystic fibrosis, both with evidence of liver disease based upon laboratory and clinical evaluation, as well as patients who did not have evidence of liver disease. This three-center study (with ours being the coordinating center) is designed to perform serial measurements of standard liver function tests and serum bile acids to determine the potential utility of serum bile acids as indicators of early liver disease.